Touch Discrimination Research Articles

Same Intervention–Different Reorganization

Background. The brain may reorganize to optimize stroke recovery. Yet relatively little is known about neural correlates of training-facilitated recovery, particularly after loss of body sensations. Objective. Our aim was to characterize changes in brain activation following clinically effective touch discrimination training in stroke patients with somatosensory loss after lesions of primary/secondary somatosensory cortices or thalamic/capsular somatosensory regions using functional magnetic resonance imaging (fMRI). Methods. Eleven stroke patients with somatosensory loss, 7 with lesions involving primary (S1) and/or secondary (S2) somatosensory cortex (4 male, 58.7 ± 13.3 years) and 4 with lesions primarily involving somatosensory thalamus and/or capsular/white matter regions (2 male, 58 ± 8.6 years) were studied. Clinical and MRI testing occurred at 6 months poststroke (preintervention), and following 15 sessions of clinically effective touch discrimination training (postintervention). Results. Improved touch discrimination of a magnitude similar to previous clinical studies and approaching normal range was found. Patients with thalamic/capsular somatosensory lesions activated preintervention in left ipsilesional supramarginal gyrus, and postintervention in ipsilesional insula and supramarginal gyrus. In contrast, those with S1/S2 lesions did not show common activation preintervention, only deactivation in contralesional superior parietal lobe, including S1, and cingulate cortex postintervention. The S1/S2 group did, however, show significant change over time involving ipsilesional precuneus. This change was greater than for the thalamic/capsular group (P = .012; d = −2.43; CI = −0.67 to −3.76). Conclusion. Different patterns of change in activation are evident following touch discrimination training with thalamic/capsular lesions compared with S1/S2 cortical somatosensory lesions, despite common training and similar improvement.

Restoring Haptic Feedback in NOTES Procedures with a Novel Wireless Tissue Stiffness Probe

In the past two decades, several instruments have been developed to overcome the loss of haptic sensation in minimally invasive surgery (MIS). Unfortunately, none of the proposed instruments has been clinically adopted or utilized in natural orifice translumenal endoscopic surgery (NOTES) procedures. The challenge is that NOTES instruments require mounting upon flexible endoscopes thus altering endoscope flexibility and dexterity. We have developed a novel wireless tissue stiffness probe (WTSP) that can be used with a flexible endoscope and create a real-time stiffness distribution map with potential to restore haptic sensation in NOTES. The aim of our study was to assess the performance and feasibility of the WTSP in an ex vivo trial (three phantom models of different elasticity; comparing discrimination of human touch with the WTSP) and in an in vivo trans-colonic access NOTES procedure. Overall, the WTSP was able to detect the stiffness of the three phantoms with a relative error smaller than 3% and a success rate of 100% versus 95% when compared to human perception. The novel WTSP was successful in providing the operator with tactile and kinesthetic feedback for accurate discrimination between tissue phantoms. In vivo tissue palpation was feasible using the WTSP in a trans-colonic NOTES procedure. The WTSP did not encumber the maneuverability or dexterity of the flexible endoscope. This innovative approach to tissue palpation has the potential to open a new paradigm in the field of NOTES where no mechanical link between the external platform and the target region exists.

Encoding of Touch Intensity But Not Pleasantness in Human Primary Somatosensory Cortex.

Growing interest in affective touch has delineated a neural network that bypasses primary somatosensory cortex (S1). Several recent studies, however, have cast doubt on the segregation of touch discrimination and affect, suggesting that S1 also encodes affective qualities. We used functional magnetic resonance imaging (fMRI) and repetitive transcranial magnetic stimulation (rTMS) to examine the role of S1 in processing touch intensity and pleasantness. Twenty-six healthy human adults rated brushing on the hand during fMRI. Intensity ratings significantly predicted activation in S1, whereas pleasantness ratings predicted activation only in the anterior cingulate cortex. Nineteen subjects also received inhibitory rTMS over right hemisphere S1 and the vertex (control). After S1 rTMS, but not after vertex rTMS, sensory discrimination was reduced and subjects with reduced sensory discrimination rated touch as more intense. In contrast, rTMS did not alter ratings of touch pleasantness. Our findings support divergent neural processing of touch intensity and pleasantness, with affective touch encoded outside of S1. Growing interest in affective touch has identified a neural network that bypasses primary somatosensory cortex (S1). Several recent studies, however, cast doubt on the separation of touch discrimination and affect. We used functional magnetic resonance imaging and repetitive transcranial magnetic stimulation to demonstrate the representation of touch discrimination and intensity in S1, but the representation of pleasantness in the anterior cingulate cortex, not S1. Our findings support divergent neural processing of touch intensity and pleasantness, with affective touch encoded outside of S1. Our study contributes to growing delineation of the affective touch system, a crucial step in understanding its dysregulation in numerous clinical conditions such as autism, eating disorders, depression, and chronic pain.

ALE meta-analysis reveals dissociable networks for affective and discriminative aspects of touch.

Emotionally‐laden tactile stimulation—such as a caress on the skin or the feel of velvet—may represent a functionally distinct domain of touch, underpinned by specific cortical pathways. In order to determine whether, and to what extent, cortical functional neuroanatomy supports a distinction between affective and discriminative touch, an activation likelihood estimate (ALE) meta‐analysis was performed. This meta‐analysis statistically mapped reported functional magnetic resonance imaging (fMRI) activations from 17 published affective touch studies in which tactile stimulation was associated with positive subjective evaluation (n = 291, 34 experimental contrasts). A separate ALE meta‐analysis mapped regions most likely to be activated by tactile stimulation during detection and discrimination tasks (n = 1,075, 91 experimental contrasts). These meta‐analyses revealed dissociable regions for affective and discriminative touch, with posterior insula (PI) more likely to be activated for affective touch, and primary somatosensory cortices (SI) more likely to be activated for discriminative touch. Secondary somatosensory cortex had a high likelihood of engagement by both affective and discriminative touch. Further, meta‐analytic connectivity (MCAM) analyses investigated network‐level co‐activation likelihoods independent of task or stimulus, across a range of domains and paradigms. Affective‐related PI and discriminative‐related SI regions co‐activated with different networks, implicated in dissociable functions, but sharing somatosensory co‐activations. Taken together, these meta‐analytic findings suggest that affective and discriminative touch are dissociable both on the regional and network levels. However, their degree of shared activation likelihood in somatosensory cortices indicates that this dissociation reflects functional biases within tactile processing networks, rather than functionally and anatomically distinct pathways. Hum Brain Mapp 37:1308‐1320, 2016. © 2016 Wiley Periodicals, Inc.

Sensory qualities of the phantom hand map in the residual forearm of amputees.

Most amputees experience referred sensations, known as a phantom hand map, on the residual forearm, where touch on specific areas is perceived as touch on the amputated hand. The aim of this study was to evaluate the sensory qualities of the phantom hand map. In 10 traumatic forearm-amputees touch thres-holds and discriminative touch of the phantom hand map were assessed and compared with corresponding areas on the contralateral forearm. The study assessed the localization of touch on the phantom hand map, and how distinct and similar to normal touch the referred feeling was. Similar touch thresholds were seen in the phantom hand map and the control site. Tactile discrimination, requiring both detection of stimulus and interpretation, was significantly better in the phantom hand map. This explorative study suggests that the phantom hand map and the superior tactile discrimination seen in the phantom hand map are based on adaptations within the brain. Further studies investigating the neural basis for the phantom hand map are needed.

Somatosensory Rehabilitation for Neuropathic Pain in Burn Survivors: A Case Series.

Neuropathic pain is an enormous rehabilitation challenge that has a substantial negative effect on patient function and quality of life. Somatosensory rehabilitation is a novel, nonpharmacological intervention described by Spicher based on the neuroplasticity of the somatosensory system. The rationale for somatosensory rehabilitation is that treating hypoesthesia will decrease neuropathic pain. Particularly for those with established neuropathic pain, the hypoesthesia may be masked by mechanical allodynia, which must be treated before treating the underlying hyposensitive zone. This case series describes the outcome of 17 burn survivors treated with somatosensory rehabilitation for their neuropathic pain. Before initiating treatment a modified version of the McGill Pain Questionnaire-short form (Questionnaire de la douleur St. Antoine, QDSA) was completed with the patients. The total score (×/64) was converted to percentage. The mechanical allodynia was assessed with the Rainbow Pain Scale that uses touch with the 15-g Semmes Weinstein Monofilaments (SWMs) and that was rated as painful on the visual analog scale (3/10 or resting pain + 1/10), as the criteria for mechanical allodynia. The severity level was assessed using seven predetermined SWMs to identify the smallest that elicited pain. The treatment consisted of avoiding all touch in the allodynic zone while concurrently providing proximal sensory and vibratory counter stimulation. Once the mechanical allodynia was eliminated, the underlying hypoesthesia was treated. Hypoesthesia was evaluated with the SWMs, and the percent improvement from baseline was calculated. The sensory reeducation treatment for hypoesthesia consisted of touch discrimination, texture perception, and vibratory stimulation. Seventeen patients (71/29% male/female, 21 ± 25% TBSA burned, 486 ± 596 days postburn) were evaluated and treated. Of these 15 initially presented with mechanical allodynia. The SWM scores had improved by 27 ± 21% (n = 14) and 29 ± 26% (n = 12) at 2 and 3 months posttreatment, respectively. The QDSA scores had improved by 9 ± 14% (n = 8) and 23 ± 23% (n = 6) at 2 and 3 months posttreatment, respectively. There were two patients who initially presented with hypoesthesia and six who had their zone of hypoesthesia treated after the mechanical allodynia had resolved. For these eight patients, their ability to perceive light touch improved by 27 ± 17% (n = 8) and 35 ± 25% (n = 6) at 2 and 3 months postsensory reeducation treatment initiation, respectively. The QDSA improved by 9 and 50% for the two patients who initially presented with hypoesthesia. In this case series, the majority of patients (13/17 or 76%) showed substantial improvements after somatosensory rehabilitation suggesting this is a treatment approach that should be considered with burn survivors experiencing neuropathic pain. There is a need, however, for future controlled studies to further investigate this approach and to determine if there is a subpopulation of burn survivors that are more likely than others to benefit from this approach.

Deaf, blind or deaf-blind: Is touch enhanced?

When someone looses one type of sensory input, s/he may compensate by using the sensory information conveyed by other senses. To verify whether loosing a sense or two has consequences on a spared sensory modality, namely touch, and whether these consequences depend on the type of sensory loss, we investigated the effects of deafness and blindness on temporal and spatial tactile tasks in deaf, blind and deaf-blind people. Deaf and deaf-blind people performed the spatial tactile task better than the temporal one, while blind and controls showed the opposite pattern. Deaf and deaf-blind participants were impaired in temporal discrimination as compared to controls, while deaf-blind individuals outperformed blind participants in the spatial tactile task. Overall, sensory-deprived participants did not show an enhanced tactile performance. We speculate that discriminative touch is not so relevant in humans, while social touch is. Probably, more complex tactile tasks would have revealed an increased performance in sensory-deprived people.

Improvement in Touch Sensation after Stroke is Associated with Resting Functional Connectivity Changes.

Distributed brain networks are known to be involved in facilitating behavioral improvement after stroke, yet few, if any, studies have investigated the relationship between improved touch sensation after stroke and changes in functional brain connectivity. We aimed to identify how recovery of somatosensory function in the first 6 months after stroke was associated with functional network changes as measured using resting-state connectivity analysis of functional magnetic resonance imaging (fMRI) data. Ten stroke survivors underwent clinical testing and resting-state fMRI scans at 1 and 6 months post-stroke. Ten age-matched healthy participants were included as controls. Patients demonstrated a wide range of severity of touch impairment 1 month post-stroke, followed by variable improvement over time. In the stroke group, significantly stronger interhemispheric functional correlations between regions of the somatosensory system, and with visual and frontal areas, were found at 6 months than at 1 month post-stroke. Clinical improvement in touch discrimination was associated with stronger correlations at 6 months between contralesional secondary somatosensory cortex (SII) and inferior parietal cortex and middle temporal gyrus, and between contralesional thalamus and cerebellum. The strength of connectivity between somatosensory regions and distributed brain networks, including vision and attention networks, may change over time in stroke survivors with impaired touch discrimination. Connectivity changes from contralesional SII and contralesional thalamus are associated with improved touch sensation at 6 months post-stroke. These functional connectivity changes could represent future targets for therapy.

Improvement of hand sensibility resulting from application of anaesthetic cream on the forearm: importance of dose and time

Introduction A local anaesthetic cream, EMLA®, applied temporarily to the forearm can improve sensory functions in the hands, both in healthy individuals and in patients with nerve injury. The treatment concept is an example of guided plasticity where the dynamic capacity of the central nervous system is used for therapeutic purposes. However, the optimal dose and duration of the anaesthetic cream is not known and this is addressed here. Methods Sixty healthy volunteers participated in this experimental study. They were randomised to one of six groups of 10 and received either 10 g or 20 g of EMLA® on the volar part of the forearm for either 60, 90 or 120 min, respectively. Outcome measures were touch thresholds and discriminative touch in digits II and V. Results There was a statistically significant improvement in touch threshold as well as discriminative touch in all six groups. However, there were no statistically significant differences between the six different dose and duration combinations. Conclusions It is concluded that 10 g of EMLA® applied to the volar part of the forearm for 60 min may be adequate to induce improved hand sensibility.

Chemotherapy-induced peripheral neuropathy.

Chemotherapy-induced peripheral neuropathy (CIPN) is common in patients receiving anticancer treatment and can affect survivability and long-term quality of life of the patient following treatment. The symptoms of CIPN primarily include abnormal sensory discrimination of touch, vibration, thermal information, and pain. There is currently a paucity of pharmacological agents to prevent or treat CIPN. The lack of efficacious therapeutics is due, at least in part, to an incomplete understanding of the mechanisms by which chemotherapies alter the sensitivity of sensory neurons. Although the clinical presentation of CIPN can be similar with the various classes of chemotherapeutic agents, there are subtle differences, suggesting that each class of drugs might induce neuropathy via different mechanisms. Multiple mechanisms have been proposed to underlie the development and maintenance of neuropathy; however, most pharmacological agents generated from preclinical experiments have failed to alleviate the symptoms of CIPN in the clinic. Further research is necessary to identify the specific mechanisms by which each class of chemotherapeutics induces neuropathy.

Enhanced early sensory outcome after nerve repair as a result of immediate post-operative re-learning: a randomized controlled trial.

We assessed the use of guided plasticity training to improve the outcome in the first 6 months after nerve repair. In a multicentre randomized controlled trial, 37 adults with median or ulnar nerve repair at the distal forearm were randomized to intervention, starting the first week after surgery with sensory and motor re-learning using mirror visual feedback and observation of touch, or to a control group with re-learning starting when reinnervation could be detected. The primary outcome at 3 and 6 months post-operatively was discriminative touch (shape texture identification test, part of the Rosen score). At 6 months, discriminative touch was significantly better in the early intervention group. Improvement of discriminative touch between 3 and 6 months was also significantly greater in that group. There were no significant differences in motor function, pain or in the total score. We conclude that early re-learning using guided plasticity may have a potential to improve the outcomes after nerve repair. LEVEL OF EVIDENCE II.

Differential recovery of touch thresholds and discriminative touch following nerve repair with focus on time dynamics

Introduction The purpose of this secondary analysis of pooled data from two randomised controlled trials was to explore the differential rate of recovery of sensory and motor functions over time following repair of median or ulnar nerve. Methods Recovery over two years following median or ulnar nerve repair at wrist level using the Rosen score was analysed in 67 patients. Results Within the sensory domain of the Rosen score, a substantial gap was observed between recovery of touch thresholds and discriminative touch. Within the motor domain, manual muscle strength and grip strength showed a closer and more parallel recovery rate. Conclusion The differential recovery rates in touch thresholds and discriminative touch after a peripheral nerve injury are likely due to neurobiological factors that cannot be influenced by surgical interventions. However, new knowledge about brain plasticity opens up the possibility that this differential recovery may diminish through the use of revised rehabilitation programs focused on the use of guided plasticity, and the timing of onset of sensory re-learning.

Discriminative and affective touch: sensing and feeling.

The multimodal properties of the human somatosensory system continue to be unravelled. There is mounting evidence that one of these submodalities-touch-has another dimension, providing not only its well-recognized discriminative input to the brain, but also an affective input. It has long been recognized that touch plays an important role in many forms of social communication and a number of theories have been proposed to explain observations and beliefs about the "power of touch." Here, we propose that a class of low-threshold mechanosensitive C fibers that innervate the hairy skin represent the neurobiological substrate for the affective and rewarding properties of touch.

Epidermal Merkel cells are mechanosensory cells that tune mammalian touch receptors.

Touch submodalities, such as flutter and pressure, are mediated by somatosensory afferents whose terminal specializations extract tactile features and encode them as action potential trains with unique activity patterns1. Whether non-neuronal cells tune touch receptors through active or passive mechanisms is debated. Terminal specializations are thought to function as passive mechanical filters analogous to the cochlea’s basilar membrane, which deconstructs complex sounds into tones that are transduced by mechanosensory hair cells. The model that cutaneous specializations are merely passive has been recently challenged because epidermal cells express sensory ion channels and neurotransmitters2,3; however, direct evidence that epidermal cells excite tactile afferents is lacking. Epidermal Merkel cells display features of sensory receptor cells4,5 and make “synapse-like” contacts5,6 with slowly adapting type I (SAI) afferents7–9. These complexes, which encode spatial features such as edges and texture1, localize to skin regions with high tactile acuity, including whisker follicles, fingertips and touch domes. Here, we show that Merkel cells actively participate in touch reception in mice. First, Merkel cells display fast, touch-evoked mechanotransduction currents. Second, optogenetic approaches in intact skin show that Merkel cells are both necessary and sufficient for sustained action-potential firing in tactile afferents. Third, recordings from touch-dome afferents lacking Merkel cells demonstrate that Merkel cells confer high-frequency responses to dynamic stimuli and enable sustained firing. These data are the first to directly demonstrate a functional, excitatory connection between epidermal cells and sensory neurons. Together, these findings indicate that Merkel cells actively tune mechanosensory responses to facilitate high spatio-temporal acuity. Moreover, our results suggest a division of labour in the Merkel cell-neurite complex: Merkel cells signal static stimuli, such as pressure, whereas sensory afferents transduce dynamic stimuli, such as moving gratings. Thus, the Merkel-cell neurite complex is unique sensory structure with two receptor cell types specialized for distinct elements of discriminative touch.

Dual Innervation of Neonatal Merkel Cells in Mouse Touch Domes

Merkel cell-neurite complexes are specialized mechanosensory end organs that mediate discriminative touch sensation. It is well established that type I slowly adapting (SAI) mechanoreceptors, which express neural filament heavy chain (NFH), innervate Merkel cells. It was previously shown that neurotrophic factor NT3 and its receptor TrkC play crucial roles in controlling touch dome Merkel cell innervation of NFH+ fibers. In addition, nerve fibers expressing another neurotrophic tyrosine receptor kinase (NTRK), Ret, innervate touch dome Merkel cells as well. However, the relationship between afferents responsive to NT3/TrkC signaling and those expressing Ret is unclear. It is also controversial if these Ret+ fibers belong to the early or late Ret+ DRG neurons, which are defined based on the co-expression and developmental dependence of TrkA. To address these questions, we genetically traced Ret+ and TrkC+ fibers and analyzed their developmental dependence on TrkA. We found that Merkel cells in neonatal mouse touch domes receive innervation of two types of fibers: one group is Ret+, while the other subset expresses TrkC and NFH. In addition, Ret+ fibers depend on TrkA for their survival and normal innervation whereas NFH+ Merkel cell innervating fibers are almost unaltered in TrkA mutant mice, supporting that Ret+ and NFH+/TrkC+ afferents are two distinct groups. Ret signaling, on the other hand, plays a minor role for the innervation of neonatal touch domes. In contrast, Merkel cells in the glabrous skin are mainly contacted by NFH+/TrkC+ afferents. Taken together, our results suggest that neonatal Merkel cells around hair follicles receive dual innervation while Merkel cells in the glabrous skin are mainly innervated by only SAI mechanoreceptors. In addition, our results suggest that neonatal Ret+ Merkel cell innervating fibers most likely belong to the late but not early Ret+ DRG neurons.

Quantifying the sensory and emotional perception of touch: differences between glabrous and hairy skin

The perception of touch is complex and there has been a lack of ways to describe the full tactile experience quantitatively. Guest et al. (2011) developed a Touch Perception Task (TPT) in order to capture such experiences, and here we used the TPT to examine differences in sensory and emotional aspects of touch at different skin sites. We compared touch on three skin sites: the hairy arm and cheek, and the glabrous palm. The hairy skin contains C-tactile (CT) afferents, which play a role in affective touch, whereas glabrous skin does not contain CT afferents and is involved in more discriminative touch. In healthy volunteers, three different materials (soft brush, sandpaper, fur) were stroked across these skin sites during self-touch or experimenter-applied touch. After each stimulus, participants rated the tactile experience using descriptors in the TPT. Sensory and emotional descriptors were analyzed using factor analyses. Five sensory factors were found: Texture, Pile, Moisture, Heat/Sharp and Cold/Slip, and three emotional factors: Positive Affect, Arousal, and Negative Affect. Significant differences were found in the use of descriptors in touch to hairy vs. glabrous skin: this was most evident in touch on forearm skin, which produced higher emotional content. The touch from another was also judged as more emotionally positive then self-touch, and participants readily discriminated between the materials on all factors. The TPT successfully probed sensory and emotional percepts of the touch experience, which aided in identifying skin where emotional touch was more pertinent. It also highlights the potentially important role for CTs in the affective processing of inter-personal touch, in combination with higher-order influences, such as through cultural belonging and previous experiences.

Self-reported Pleasantness Ratings and Examiner-Coded Defensiveness in Response to Touch in Children with ASD: Effects of Stimulus Material and Bodily Location

Tactile defensiveness, characterized by behavioral hyperresponsiveness and negative emotional responses to touch, is a common manifestation of aberrant sensory processing in autism spectrum disorders (ASD) and other developmental disabilities (DD). Variations in tactile defensiveness with the properties of the stimulus and the bodily site of stimulation have been addressed in adults with self-report of perceived tactile pleasantness, but not in children. We presented three materials (pleasant, unpleasant, social) at three bodily sites and measured both examiner-coded defensiveness and self-reported pleasantness from a group of children with ASD and two comparison groups (one with DD, one with typical development (TD)). The main findings were: (1) children with ASD and DD showed significantly more defensiveness reactions and lower pleasantness ratings than the TD group, with higher variability, (2) there was a double dissociation for the effects of material and bodily site of stimulation: while bodily site predicted behavioral defensiveness, material predicted pleasantness rating. Additionally, it was noted that (3) the most pleasant material and the social touch conditions best distinguished ASD and DD from TD on defensiveness, and (4) within the ASD group, social impairment and defensiveness in bodily sites associated with social touch were positively correlated, suggesting a clinically relevant distinction between social and discriminative touch in ASD.

A closed-loop neurobotic system for fine touch sensing

Objective. Fine touch sensing relies on peripheral-to-central neurotransmission of somesthetic percepts, as well as on active motion policies shaping tactile exploration. This paper presents a novel neuroengineering framework for robotic applications based on the multistage processing of fine tactile information in the closed action–perception loop. Approach. The integrated system modules focus on (i) neural coding principles of spatiotemporal spiking patterns at the periphery of the somatosensory pathway, (ii) probabilistic decoding mechanisms mediating cortical-like tactile recognition and (iii) decision-making and low-level motor adaptation underlying active touch sensing. We probed the resulting neural architecture through a Braille reading task. Main results. Our results on the peripheral encoding of primary contact features are consistent with experimental data on human slow-adapting type I mechanoreceptors. They also suggest second-order processing by cuneate neurons may resolve perceptual ambiguities, contributing to a fast and highly performing online discrimination of Braille inputs by a downstream probabilistic decoder. The implemented multilevel adaptive control provides robustness to motion inaccuracy, while making the number of finger accelerations covariate with Braille character complexity. The resulting modulation of fingertip kinematics is coherent with that observed in human Braille readers. Significance. This work provides a basis for the design and implementation of modular neuromimetic systems for fine touch discrimination in robotics.

Neuronal correlates of tactile speed in primary somatosensory cortex

Moving stimuli activate all of the mechanoreceptive afferents involved in discriminative touch, but their signals covary with several parameters, including texture. Despite this, the brain extracts precise information about tactile speed, and humans can scale the tangential speed of moving surfaces as long as they have some surface texture. Speed estimates, however, vary with texture: lower estimates for rougher surfaces (increased spatial period, SP). We hypothesized that the discharge of cortical neurons playing a role in scaling tactile speed should covary with speed and SP in the same manner. Single-cell recordings (n = 119) were made in the hand region of primary somatosensory cortex (S1) of awake monkeys while raised-dot surfaces (longitudinal SPs, 2-8 mm; periodic or nonperiodic) were displaced under their fingertips at speeds of 40-105 mm/s. Speed sensitivity was widely distributed (area 3b, 13/25; area 1, 32/51; area 2, 31/43) and almost invariably combined with texture sensitivity (82% of cells). A subset of cells (27/64 fully tested speed-sensitive cells) showed a graded increase in discharge with increasing speed for testing with both sets of surfaces (periodic, nonperiodic), consistent with a role in tactile speed scaling. These cells were almost entirely confined to caudal S1 (areas 1 and 2). None of the speed-sensitive cells, however, showed a pattern of decreased discharge with increased SP, as found for subjective speed estimates in humans. Thus further processing of tactile motion signals, presumably in higher-order areas, is required to explain human tactile speed scaling.

Integration of Sensory Quanta in Cuneate Nucleus Neurons In Vivo

Discriminative touch relies on afferent information carried to the central nervous system by action potentials (spikes) in ensembles of primary afferents bundled in peripheral nerves. These sensory quanta are first processed by the cuneate nucleus before the afferent information is transmitted to brain networks serving specific perceptual and sensorimotor functions. Here we report data on the integration of primary afferent synaptic inputs obtained with in vivo whole cell patch clamp recordings from the neurons of this nucleus. We find that the synaptic integration in individual cuneate neurons is dominated by 4–8 primary afferent inputs with large synaptic weights. In a simulation we show that the arrangement with a low number of primary afferent inputs can maximize transfer over the cuneate nucleus of information encoded in the spatiotemporal patterns of spikes generated when a human fingertip contact objects. Hence, the observed distributions of synaptic weights support high fidelity transfer of signals from ensembles of tactile afferents. Various anatomical estimates suggest that a cuneate neuron may receive hundreds of primary afferents rather than 4–8. Therefore, we discuss the possibility that adaptation of synaptic weight distribution, possibly involving silent synapses, may function to maximize information transfer in somatosensory pathways.